Recent advancements in the field of cellular communication include supporting direct or device-to-device (D2D) communication between two or more mobile devices operating in the licensed spectrum. Direct communication between mobile devices, with or without the coordination of the cellular communication network, provides a vast array of advantages over traditional cellular communication including improving local coverage, facilitating traffic offloading from cellular networks, providing service continuity for mobile devices that are moving away from a cellular network, supporting mission critical public safety communication and providing potential for various types of new services and applications.
Currently, the 3rd Generation Partnership Project (3GPP) is working towards standardising D2D communication in its existing long Term Evolution Advanced (LTE-A) cellular networks. It is envisioned that the upcoming Release-12 of the 3GPP standards will include D2D functionality to support public safety as well as non-public safety services. More specifically, support for D2D discovery in network coverage, D2D direct communication in and out-of network coverage and high layer support to enable group cast and unicast over the physical layer broadcast have already been standardized.
A work item for 3GPP Release 13 has been approved relating to more advanced features to support discovery in partial and outside network coverage, network coverage extension using UE-to-network relays, and discovery in the presence of multiple carriers and Public Land Mobile Networks (PLMNs). Of the objectives in the 3GPP Release-13 work item, UE-to-network relay is of particular interest in the public safety community as it may facilitate reliable public safety communication via service continuity and coverage extension. A UE-to-network relay feature is expected to be implemented in 3GPP Release-13 as Layer 3 packet forwarding based relaying supported by Release-12 D2D direct communication in the Physical layer.
3GPP Release-12 D2D direct communication is anticipated to include two operation modes, namely Mode-1 operation mode and Mode-2 operation mode. In Mode-1 operation mode, a base station (BS) schedules resources to be used by a UE to transmit direct control information followed by direct data. In particular, the BS uses a Physical Downlink Control Channel (PDCCH) or enhanced Physical Downlink Control Channel (ePDCCH) to allocate resources to a D2D transmitter for D2D control information or scheduling assignment (SA) transmission and D2D Data transmission, thus enabling contention and collision free multi-user access for Mode-1 communication. In contrast, in Mode-2 operation mode, a D2D-UE selects resources on its own from network configured or pre-configured D2D communication resource pools, for transmission of direct control information or SA and direct data. Mode-2 communication can be operated under network coverage or out-of-network coverage. However, in both scenarios, resource selection for SA and data transmission is performed by the UE on its own without a central coordination.
FIG. 1 illustrates a communication system 1 including UE-to-network relay operation. In particular, a device-to-device communication capable UE (D2D relay UE 11) that supports relay functionality, which is in the coverage of a cellular BS 12, may act as a Layer 3 (i.e. Internet Protocol) IP packet relay to another out-of coverage D2D-UE (i.e. OOC-UE 13). In this scenario, the communication between the D2D relay UE 11 and the OOC-UE 13 may take place in the physical layer using Mode-2 communication, while the communication between the D2D relay UE 11 and the BS 12 may follow cellular uplink (UL) and downlink (DL) communication procedures.
In one use case, the OOC-UE 13 is a public safety UE (PS UE) that wishes to communicate with another PS UE via a public safety network, and the OOC-UE 13 may utilise the D2D relay UE 11 for this purpose. In this scenario, the spectrum (f2) used by the OOC-UE 13 to communicate with other OOC-UEs is same as the spectrum (f1) used by the public safety network. In another use case, the D2D relay UE 11 may be a PS UE who is roaming on a commercial network; and this UE may act as a relay node to another PS UE (OOC-UE) who is outside the coverage of the commercial network. In this scenario, the spectrum (f2) used by the OOC-UE 13 to communicate with other OOC-UEs is different than the spectrum (f1) used by the commercial network.
Such D2D communication is anticipated to utilise the cellular UL resources, and operate in half-duplex manner. As a result of this single carrier operation, cellular UL transmission and D2D transmission (TX) or D2D reception (RX) cannot occur simultaneously. Therefore, from a UE perspective, cellular UL (TX) and D2D (TX/RX) operation needs to be time multiplexed. 3GPP has further chosen to prioritise cellular UL transmissions over D2D (TX/RX) communication in case of a resource collision or limited UE TX/RX capability, and not to introduce any discontinuous transmission (DTX) period for the purpose of aiding UE TX/RX switching.
Moreover, RX switching/re-tuning from one carrier to another (for instant from cellular DL to UL in FDD) may require at least one additional subframe for tuning purposes. As such, in subframes allocated for D2D, a D2D-UE with limited TX/RX capability may have to suspend D2D (TX/RX) communication in order to perform cellular communication. Since D2D transmission from the OOC-UE can be in any subframe within the Mode-2 resource pool that are also used by the relay UE in UL cellular communication (due to autonomous resource selection by the D2D-UE for Mode-2), the relay UE may miss reception of D2D data from the OOC-UE, resulting in packet loss or high latency. However, from a UE-to-network relay perspective, both cellular and D2D data is equally important to facilitate a reliable and low latency (for example VoIP) link between OOC-UE and the network.
Accordingly, there is a need for an improved method and system for data communication.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.